Water-soluble binders are useful in molding of ceramic compositions because of their advantages attributable to water solubility. While the molding process includes the step of heat drying the ceramic composition, which is loosely bound by the binding force exerted by the water-soluble binder in aqueous solution form, for increasing the bond strength to establish a stable bond, the heat drying step is just to evaporate only water. Unlike those binders in organic solvents which make it necessary to recover the solvent during the drying step for reducing any detrimental effect to the environment, the water-soluble binders dispense with such cumbersome operation.
Exemplary water-soluble binders include starch, polyacrylic acid aqueous solution, polyethylene oxide, alkyl celluloses, hydroxyalkyl celluloses, hydroxyalkyl alkyl celluloses, and the like. Of these, alkyl celluloses are in stark contrast to other water-soluble binders having the drawback that aqueous solutions thereof lose an apparent viscosity upon heating so that the bound shape may fail or alter during drying, in that the alkyl celluloses are able to maintain the bound shape because their aqueous solutions become gel upon heating. Thus drying proceeds in the state bound by the gel. This binding enables to maintain the molded shape. For this reason, the alkyl celluloses are used in ceramic molding, for example, as the binder having a shape-retaining ability during drying.
JP-A 2002-037923 discloses a binder comprising a cellulose ether whose syneresis value is increased by increasing a specific group substitution. The binder has sufficient properties to improve the drying and shape retention of ceramic compositions. JP-A 2002-047069 describes a method for producing a honeycomb structure from an electroconductive ceramic composition, using hydroxypropyl methyl cellulose having a water syneresis value of at least 40 wt %. The method ensures effective drying and inhibits cracking due to shrinkage strain during drying.
The method of JP-A 2002-047069, however, has a problem when it is desired to manufacture a honeycomb shaped part having a thinner wall or a higher porogen content. Albeit effective drying due to high syneresis, shape retention during drying (or shrinkage) is insufficient. Deformation or cracking can occur during drying. This phenomenon becomes outstanding particularly with electroconductive ceramic compositions. It would thus be desirable to have a water-soluble binder having a good profile of rapid drying and shape retention due to heat gelation during drying, and to prepare such a binder at low costs including raw material costs.